Mark Zuckerberg was conspicuously absent at this week's congressional hearings on how Russia exploited U.S. social media to sow discord during the 2016 election. But the Facebook CEO says he's "dead serious" about fixing the problem.

To prove it, Facebook will "invest so much in security" that the costs will cut into the company's profitability, Zuckerberg said in an earnings call.

"I've expressed how upset I am that the Russians tried to use our tools to sow mistrust," he said on Wednesday. "They used them to try to undermine our values. What they did is wrong and we're not going to stand for it."

Zuckberg made the remarks after some U.S. senators slammed the company for failing to stop Facebook from becoming a propaganda tool of the Russian government. At Wednesday's hearing, U.S. senators showed examples of how the Kremlin allegedly used Facebook ads and posts to influence last year's presidential election and divide American society. Among them was a successful attempt to trick pro-Texas Facebook users and Muslims to clash outside an Islamic Center in Houston last year.

However, only legal counsel from Facebook, Google and Twitter attended the three hearings that took place on Capitol Hill this week, which disappointed some of the lawmakers present.

"You have a huge problem on your hands," said Sen. Dianne Feinstein of California. "And you have to be the ones to do something about it -- or we will."

A year ago, Zuckerberg said it was "crazy" that fake news on Facebook influenced the presidential election, but he has since changed his tone. "We're bringing the same intensity to these security issues that we've brought to any adversary or challenge we've faced," he said on Wednesday.

Facebook already has 10,000 staffers working on "safety and security," but plans on doubling that figure by next year, Zuckerberg added. "We're also building new AI to detect bad content and bad actors -- just like we've done with terrorist propaganda," he said.

Facebook has also announced plans to make political ads on the platform more transparent by showing users who sponsored them. But whether any of these efforts will be enough to placate U.S. lawmakers remains to be seen.